Field of the invention
The subject disclosure relates to an electrophotographic image forming apparatus.
Description of the Related Art
In an electrophotographic image forming apparatus, after a surface of an image bearing member such as a photosensitive drum is charged, toner images formed through an exposure process and a development process are transferred to a recording medium directly or indirectly through an intermediate transfer member to form an image on the recording medium. A charging process and a transferring process are executed when a bias voltage is applied from a high-voltage circuit board to a charging member and a transfer member that face the image bearing member. In many cases, output of a charging bias is started before output of a transfer bias is started, and the output of the transfer bias is started while the charging bias is stable. This is because a surface potential in a partial region of the photosensitive drum is prevented from changing to a polarity opposite to a polarity of the charging bias due to a transfer current flowing between the photosensitive drum and the transfer member by the transfer bias. If the surface potential of the photosensitive drum is changed to the polarity opposite to the polarity of the charging bias, charging potential unevenness (charging unevenness) occurs in the following charging process, which may cause image defect such as horizontal stripes.
On the other hand, in the image forming apparatus, a time period after printing start operation such as pressing of a copy button is performed by a user until a first recording medium on which the image has been formed is discharged, is called a first copy output time (FCOT). It is desired to reduce the FCOT as much as possible because the user waits for the time period of the FCOT.
Japanese Patent Application Laid-Open No. 2014-170156 discusses an image forming apparatus in which output of the transfer bias is started at a time when a surface position of the photosensitive drum at which the charging by the charging roller is started reaches a transfer position by a primary transfer roller, thereby reducing the FCOT. The image forming apparatus controls the output of the transfer bias through pulse width modulation, and a duty radio of the transfer bias is set to a value smaller than a regular set value during a predetermined period at the beginning of raising. This prevents a transfer current from overshooting in raising of the transfer bias, and to prevent the charging unevenness from remaining as a history of the overshooting in the following charging process.
In the transfer member such as the transfer roller and the charging member such as the charging roller, impedance is varied due to difference of conditions such as use environment and a cumulative use time. Accordingly, a time after the output of the charging bias and the output of the primary transfer bias are started until the output reaches a target voltage value or a target current value is different depending on the conditions. The configuration discussed in Japanese Patent Application Laid-Open No. 2014-170156, however, is not made in consideration of such condition difference, and the charging unevenness accordingly occurs in some cases. In other words, in a case where a rising speed of the charging bias is relatively low and a rising speed of the transfer bias is relatively high, an excess transfer current may flow through the surface region of the photosensitive drum that has not been sufficiently charged, which causes the surface potential to become the polarity opposite to the polarity of the charging bias.